Melamine, whose structure is represented by the formula I,
is used for preparing melamine resins by reaction with carbonyl-containing compounds. The resins are used, inter alia, as plastics and in paints and varnishes. The preparation of melamine by decomposition of urea is a known reaction which is utilized in a number of variants by the chemical industry. A distinction is made in principle between the high-pressure process and the low-pressure process. The high-pressure process is carried out at pressures of >about 80 bar (abs.) and temperatures of >370° C. in the absence of catalysts.
However, the low-pressure process which is carried out at pressures from about 1 to 10 bar (abs.) and temperatures of from 370 to 430° C. is of greater importance in the context of the present invention. It is known that the reaction proceeds in two steps. In the first, endothermic step, urea reacts to form ammonia and isocyanic acid which trimerizes in a second, exothermic step to form melamine and liberate CO2. The following equations describe the individual reactions.6H2N—C(O)—NH2→6 HN═C═O+6NH3□H=984 kJ/mol6HN═C═O→C3N3(NH2)3+3CO2□H=−355 kJ/mol6H2N—C(O)—NH2→C3N3(NH2)3+6 NH3+3 CO2□H=629 kJ/mol
There are three main variants of the low-pressure process, which are described in greater detail below.
In the process of Linz-Chemie, the reaction is carried out in two stages. In the first stage, molten urea is decomposed in a fluidized bed of sand at 350° C. and 3.5 bar (abs.) to form ammonia and isocyanic acid. Isocyanic acid is subsequently converted catalytically into melamine at 450° C. and atmospheric pressure in a fixed-bed reactor. The catalyst is generally an aluminum oxide catalyst.
The DSM-Stamicarbon process is a single-stage process carried out at about 7 bar (abs.). Catalysts used are aluminum silicates which are employed as a fluidized bed. The fluidizing gas is pure ammonia which is recovered by work-up of the offgas.
Finally, there is the BASF process. Here too, the reaction is carried out in a fluidized bed using aluminum oxide or aluminum oxide/silicon dioxide catalysts at a low pressure (about 2 bar abs.). The gas used for the fluidized bed is recycled gas from the reactor which comprises NH3 and CO2 and has previously been freed of impurities, generally by treatment with a urea melt which takes up the impurities.
A problem which frequently occurs when carrying out all the abovementioned catalytic processes, which in principle offer the advantage of simpler, cheaper apparatuses compared to the noncatalytic processes, is the deposition of higher condensation products of melamine on the surface of the catalyst (coating). An example which may be mentioned here is melem (C6H6N10, 2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene) which is a three-ring compound made up of three fused triazine rings. The excessive deposition of condensation products is associated with deactivation of the catalyst, requiring regeneration of the catalyst, for example by thermal treatment and/or treatment with steam, air or ammonia, but in an extreme case also making replacement of the catalyst necessary. Since, in addition, deposition of condensation products on the catalyst proceeds relatively quickly to give a steady-state concentration of these products, deactivation frequently occurs after a very short period of time, so that periodic regeneration is not feasible because of the short time intervals.
There have hitherto been no systematic studies nor knowledge regarding the properties or compositions which a catalyst used in the synthesis of melamine has to have in order to achieve a high yield or a low degree of decomposition.
JP-A 08 027 126 claims a γ-Al2O3 catalyst having defined acidity limits for the synthesis of melamine.
Thianranqi Huagong, 2001, volume 26, pages 23 to 25 (cited on the basis of CA 136:135396) discloses that active catalysts for the synthesis of the melamine can be obtained by mixing Al2O3 with zeolites or zeolites containing metal cations. The activity obtained is ascribed to the acidic centers of the catalyst.
However, it has been found that the use of catalysts having an increased acidity is not able to solve the problems of catalyst deactivation, in particular as a result of deposit formation, and the low conversions associated therewith.
It is an object of the present invention to provide a process by means of which it is possible to obtain high conversions and melamine yields without premature deactivation of the catalyst as a result of deposit formation occurring, particularly under the chosen reaction conditions.
We have found that this object is achieved by a process for the catalytic preparation of melamine by decomposition of urea over solid catalysts using a main reactor and an after-reactor, wherein a catalyst having a low Lewis acidity is used in the main reactor and a catalyst having an equal or higher Lewis acidity is used in the after-reactor.